We propose color reproduction methods and computing systems for creating specularly reflecting color images printed on a metallic or metallized substrate that change their color upon in-plane rotation by 90°. Color changing metallic prints rely on the anisotropic dot gain of line halftones when viewed under specular reflection. The proposed color reproduction methods rely on a spectral prediction model specially conceived for predicting the color of non-rotated and of 90° in-plane rotated cross-halftones formed of superpositions of horizontal and vertical cyan, magenta and yellow line halftones. Desired non-rotated and rotated image colors are mapped onto the sub-gamut allowing for the desired color shift and then, using a 6D correspondence table, converted to optimal cross-halftone ink surface coverages. The proposed color color changing and decolorization techniques are especially effective for creating surprising effects such as image parts whose hues change, or gray regions that become colorful. These methods can be adapted to commercial printers capable of printing with cyan, magenta and yellow inks on substrates formed by an ink attracting plastic layer located on top of a metallic film layer. The resulting printed color varying images enable preventing counterfeits of security documents and valuable articles and at the same time create surprising effects that can be used to increase their attractiveness and aesthetics. Therefore, the invented color imaging methods are also useful for decoration, advertisement, art and amusement parks.
In respect to the prior art, U.S. Pat. No. 7,491,424 B2, Reproduction of security documents and color images with metallic inks, filed 19 May 2003, to inventors R. D. Hersch, F. Collaud, and P. Emmel, teaches a security feature obtained by combining a metallic ink and standard inks printed on paper to create color images with embedded hidden patterns under non-specular reflection. The patterns are revealed under specular reflection.
Pjanic and Hersch (also inventors in the present invention) created a color reproduction workflow for printing with classical inks on a metallic substrate, see P. Pjanic and R. D. Hersch, Specular color imaging on a metallic substrate, in Proc. IS&T 21st Color Imaging Conference, 61-68 (2013). In order to establish the correspondence between amounts of inks and resulting color under specular reflection, they used an ink spreading enhanced cellular Yule-Nielsen spectral prediction model. The goal of that method is to simply reproduce an original color image. It does not support the creation of desired color variations by rotating the print under specular reflection.
In U.S. patent application Ser. No. 14/747,184, Color reproduction, pattern hiding and image alternations with halftone prints on metal, filed 23 Jun. 2015, inventors P. Pjanic and R. D. Hersch, also inventors in the present patent application, teach methods for creating color or achromatic images that are printed with classical cyan, magenta, yellow inks and a white diffusing ink on a metallic substrate. These methods allow to view on the same metallic print one grayscale image in specular viewing mode and a second independent grayscale image in non-specular viewing mode. They also allow to hide a pattern such as text, graphics or a grayscale image within the printed color image in one viewing mode, specular or non-specular and show that pattern in the second viewing mode, non-specular or specular, respectively. In contrast to the present invention, the halftone layers are made of round ink dots and the in-plane rotation of the print under specular reflection does not induce desired color variations. That patent application appeared also as technical paper at the SIGGRAPH conference 2015, Los Angeles, that started on 9 Aug. 2015. The paper reference is the following: P. Pjanic, R. D. Hersch, Color Imaging and Pattern Hiding on a Metallic Substrate, Proc. SIGGRAPH, ACM Trans. On Graphics 34 (3), article 130, (2015). This paper was made available to the public 2 weeks before the start of the conference, from the ACM Digital Library.